Add ability to sell items back to Merchants

[UndeadZeratul]

Whether as a unique mode of acquiring currency, or as a way to supplement the existing ones, it'd be nice to be able to sell junk back to merchants to earn a few MercBucks back. Obviously the amount earned will have to be scaled way back, as the average level could contain a few hundred enemies plus map spawns, dedicated players will simply hoard everything and sell it to the merchant en masse and ruin the MercBucks economy!

Either that or Merchants randomly pick what things they're buying as well as selling, or have a starting amount of currency which goes up/down as players buy/sell stuff from/to them.